A laser beam is used in laser-annealing applications to crystallize an amorphous material to obtain a crystalline, or polycrystalline, material. For example, excimer lasers may be used to crystallize an amorphous silicon (a-Si) film to obtain a polycrystalline silicon (poly-Si) region. In several of these methods, a mask is inserted into the path of the beam to shape the laser beam before the beam irradiates the material.
The mask, used to shape the beam, can in principal have a wide variety of patterns on it. The mask may comprise a patterned layer of chrome, or other material that blocks the desired wavelength effectively, on a quartz substrate, or other suitably transparent material at the wavelength of laser to be used. Common patterns consist of groups of rectangular shapes, including slits and chevrons.
As the laser beam is projected through these patterns, the intensity profile of the projected beam will be determined by the features that make up the pattern and any optics used to image the pattern on the material. The intensity profile of the laser beam is typically not uniform over the entire pattern. For example, at corner regions intensity peaks have been noticed. These intensity peaks may cause local damage on the film irradiated by the shaped beam. One form of damage caused by intensity peaks is agglomeration, which may cause the silicon film to pull away from the region exposed to the high intensity peaks, possibly leaving a void or other non-uniformity.